I am currently stateless.
But this is a good thing. Because Japan does not recognise multiple citizenships, you are required to renounce your existing citizenship as part of the process of becoming a Japanese citizen. If your current countries of citizenship will allow you to renounce those citizenships before you gain Japanese citizenship, you must do so; the Ministry of Justice will not finalise your naturalisation without the document proving that you have renounced your other citizenships. The UK does allow you to renounce your citizenship in anticipation of gaining citizenship of another country, and so I had to.
For people who want to keep track, it took just under three months from posting the application to renounce to getting the certification back, and they did not return my old UK passport.
International law, in general, tries to avoid people being stateless for very long. That means that, as I understand it, the last stage of my naturalisation will be quick. I don’t know exactly how quick, but I guess I’m about to find out. I will take the documents to the local Legal Affairs Bureau and submit them tomorrow.
I prepared a Japanese translation of the document, although I’m not absolutely sure I needed to. While doing that I noticed the date on which I formally ceased to be a UK citizen.